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Trump has previously flown on a U.S. presidential jet once previously, when he flew from New York City to Washington, D.C. for his inauguration last week.

But the aircraft he is riding today is a different - and larger - plane. Also, a plane or helicopter can only receive the call sign of Air Force One or Marine One if the Commander in Chief is on board.

Trump is set to make his comments to the GOP at the retreat around noon.

Before the flight, White House Press Secretary Sean Spicer and the president's adviser, Kellyanne Conway, addressed the pool of reporters on board.

Despite a rocky start to his administration, many lawmakers are optimistic about delivering change in a new era of GOP control over Washington. They would like to see a Trump committed to their agenda and results, not a president who veers off course into conspiracy theories about voter fraud or who keeps litigating the size of his inaugural crowds.

Before Trump's appearance, House Speaker Paul Ryan sketched out an ambitious agenda to lawmakers that includes sending Trump a health care repeal bill by March and a rewrite of tax laws by summer's end.

Also in the first 200 days Congress intends to confront paying for Trump's newly announced border wall, which Ryan confirmed could cost $8billion to $14billion, and will work on a public works bill that Trump requested be added to a crowded agenda.

President Donald Trump gives a thumbs-up as he arrives on Air Force One, Thursday, Jan. 26, 2017, at the Philadelphia International Airport in Philadelphia, before speaking at the Republican Congressi

"I'm just so excited we finally have a chance to do this because we have the House and the Senate and a president who is with us," Ryan told MSNBC on Wednesday about plans to overhaul the tax system, eliminating what critics say are loopholes and lowering corporate rates to 20 percent or even the 15 percent sought by Trump.

"If you can clean up the cesspool of the tax code and give us a pro-growth tax code, that is how you grow the economy, that is how you take power and money out of Washington and give it back to the people," he said.

Lawmakers were generally enthusiastic to see Trump take quick action on immigration, oil pipelines and other issues via executive order, even though they criticized Barack Obama for overusing such administrative tools when he was president. This time, Republican lawmakers justify it by saying Trump, in many cases, is undoing what Obama did.

Yet there were signs that Congress might not easily go along with fronting the money for Trump's border plan, which he continues to insist Mexico will ultimately pay for, though without explaining how.

As for Trump's fixation on supposed illegal voting by 3 million to 5 million people, which is untrue, and the attendance at his inauguration: 'Those are distractions, and it's dwelled upon. I particularly don't care about it,' Cassidy said.

Most Republicans are taking Cassidy's approach of playing down the distractions. But others said there was real concern that Trump could be the GOP's own worst enemy at the very moment they've seized full control of Washington and believe they have a mandate to usher in sweeping change, starting with repealing and replacing Obama's health care law.

Lawmakers also were to hear from Vice President Mike Pence and British Prime Minister Theresa May. Lawmakers said it was a chance to showcase the relationship with Britain in the visit from May, who vaulted to power as a result of the surprise vote to leave the European Union; many saw that as a precursor to Trump's own victory.

Her speech was coming on the same day when Trump was expected to sign executive orders starting work on bilateral trade deals to take the place of the sweeping multilateral Asia-Pacific pact negotiated under the Obama administration, which Trump announced the U.S. is discarding.